Platelets are of great importance for many pathophysiological processes, including thrombosis, hemorrhage, inflammation, and cancer. The second messenger Ca2+ is critical for several facets of platelet activation. However, the nature of the molecule(s) linking calcium mobilization to the signaling pathways regulating platelet activation is largely unknown. The goal of this proposal is to establish CalDAG-GEFI (CD- GEFI) as a Ca2+ sensor that is central to integrin activation, thromboxane A2 (TxA2) generation, and granule release. CD-GEF proteins are guanine nucleotide exchange factors for Ras family small GTPases. They are regulated by both by Ca2+ and/or diacylglycerol (DAG). We have shown that CD-GEFI, the major isoform in platelets, is a central component of Ca2+-dependent activation of Rap1 and 1/3 integrins. Integrin activation in the absence of CD-GEFI required signaling by protein kinase C (PKC) and the Gai-coupled ADP receptor, P2Y12. The P2Y12 receptor is the target of one of the most successful anti-thrombotic drugs, clopidogrel. Unexpectedly, the PKC/P2Y12-dependent pathway was not able to support thrombus formation under arterial flow conditions in CD-GEFI-/- mice. With the current study, we aim to understand critical variables regulating both CD-GEFI- dependent and -independent platelet thrombosis. Three major unresolved questions will be asked. First, what is the role of CD-GEFI in Ca2+-dependent TxA2 generation and granule release, and how does it communicate with well-established signaling pathways such as PKC and PI3 kinase? It is our hypothesis that CD-GEFI directly affects TxA2 generation and ADP release through Rap1/2-mediated activation of ERK MAP kinases and the small GTPase Rac1, respectively. In platelets activated with weak agonists, CD-GEFI mediates the first wave of TxA2 release, which provides essential feedback for PKC- mediated granule release. PI3 kinase participates in CD-GEFI- and P2Y12-dependent Rap1/2 activation, depending on the agonist and mechanism of platelet activation. Second, how is CD-GEFI function regulated in platelets? We hypothesize that CD-GEFI is a high-affinity Ca2+ sensor in platelets, which does not rely on binding of DAG to its C1 domain (in contrast to other members of the CD-GEF family). We further propose that translocation of CD-GEFI to the plasma membrane during platelet activation depends on its direct association with the cytoskeleton, and that CD-GEFI serves as an adapter for Rap1/2. We will test these hypotheses by performing structure-function studies in platelets. Third, what are the conditions allowing for CD-GEFI- independent platelet adhesion and thrombus formation under flow? Using flow chamber and intravital microscopy approaches, we will test our hypothesis that CD-GEFI-independent adhesion is relevant in vivo when thrombus formation is driven by thrombin under low shear conditions, such as in venous thrombosis models. We have strong preliminary data supporting each of the above specific aims. In summary, our studies will identify CD-GEFI as a central sensor linking Ca2+ mobilization to integrin activation, TxA2 generation, and granule release. An in-depth analysis of how CD-GEFI regulates platelet function in vitro and in vivo will aid in its establishment as a target for antiplatelet therapy.